The present invention concerns an arrangement, which includes a scope, such as a rigid borescope, a flexible fibrescope, or a videoscope for example, for accurately measuring observed objects, object details, and object defects. The arrangement of the present invention is more accurate than known systems, yet is simple to use.
Scopes, such as flexible videoscopes and fibrescopes have been used to observe the interior of the body during diagnostic procedures or surgery. Scopes, such as rigid borescopes, have been used to observe and inspect manufactured parts otherwise inaccessible to the eye. Although such scopes have an almost limitless number of applications, the following example illustrates their value.
A gas turbine engine includes a series of compressor and turbine blades, any one of which may become damaged. Although the first and last stage compressor/turbine blades of a gas turbine engine can be inspected directly, other intermediate stage compressor/turbine blades cannot be directly inspected. In the past, to inspect these intermediate stage compressor/turbine blades, the engine had to be disassembled until the intermediate stage compressor/turbine blade could be directly inspected. However, more recent gas turbine engines are with apertures (or borescope ports) provided at critical areas. These borescope ports permit the intermediate stage blades to be inspected using a borescope.
The borescope includes a long, thin, insertion tube having a lens system at its distal end and a viewing means at its proximal end. When the insertion tube of the borescope is inserted into a borescope port of the gas turbine engine, the lens system at its distal end relays an image of an otherwise inaccessible intermediate compressor/turbine blade to the viewing means at the proximal end. The focus of the image (in some models) can be adjusted by control knobs at the proximal end of the borescope. Hence, as illustrated by this example, a borescope permits an intermediate compressor/turbine blade of a gas turbine engine to be inspected without needing to disassemble the engine.
Besides being used to indirectly inspect parts which cannot be inspected directly, borescopes can also be used to measure the size of defects on the part. For example, U.S. Pat. No. 4,980,763 (hereinafter "the '763 patent") discusses a system for measuring objects viewed through a borescope. The system discussed in the '763 patent projects an auxiliary image, such as a shadow, onto the object being viewed. Changes in the position or size of the auxiliary image correspond to the distance between the object being viewed and the borescope. The image is displayed on a monitor having a magnification and object distance scale overlay screen. The size of the object on the screen is measured with vernier calipers, or electronically with cursors. This size is then divided by the magnification which is determined by observing where the auxiliary image falls on the magnification overlay. Unfortunately, the system discussed in the '763 patent requires a user to manually determine the magnification factor based on the position of the auxiliary image on the display screen.
U.S. Pat. No. 4,207,594 (hereinafter "the '594 patent") discusses a system in which the dimensions of a defect are determined based upon a manually entered field-of-view value and a ratio of second crosshairs, arranged at edges of a defect image, to first crosshairs, arranged at edges of the field of view. Unfortunately, the system discussed in the '594 patent requires probe penetration values to be manually read from a scale on the probe barrel for determining the field-of-view. Since such scales do not have fine gradations and since they must be manually read, errors are introduced.
U.S. Pat. No. 4,820,043 (hereinafter "the '043 patent") discusses a technoscope for determining the length of a defect. The technoscope includes a graduated scale which is displacable in a direction transverse to the endoscope axis. The graduated scale is mechanically coupled with a detector which produces an electrical signal based on the transverse displacement of the graduated scale. The distance to the defect is determined by (i) observing the object image at a first terminal position of a fixed stroke Z of the endoscope, (ii) noting the intercept of the object image on the graduated scale, (iii) axially displacing the endoscope by the fixed stroke Z, and (iv) transversely displacing the graduated scale until the defect image intercepts it at the same point as before the axial displacement. A calculator uses the electrical signal from the detector and a known focal length of endoscope to determine the object distance. The size of the defect can be similarly determined. The technoscope of the '043 patent also includes a swing prism with a detector for determining its angular position.
Unfortunately, the scone of the '043 patent requires that the Local length of the endoscope be known ahead of time and requires two measurements. Moreover, since the distance between the two measurements must be fixed, the scone must be fixed with respect to the object during the two measurements. Furthermore, limitations in the gradations of the graduated scale limits the accuracy of the readings. Also, by manually reading the intercept point of the defect on the graduated scale, errors are introduced.
U.S. Pat. No. 4,702,229 (hereinafter "the '223 patent") discusses a technoscope for measuring an object. The technoscope includes an inner shaft which is axially displaceable with respect to an outer shaft. A measuring scale is provided in the inner shaft. The measurement of the object is determined by (i) placing an edge of the object image on the measuring scale, (ii) fixing the technoscope with respect to the object, (iii) axially displacing the inner shaft by a fixed distance, and (iv) observing how many scaler divisions the object image moved on the measurement scale. The object size is determined based on a known system focal length, the length of the displacement, and the number of scales moved by the object. The '229 patent is similar to the '043 patent except that with the '043 patent, the graduate scale is transversely repositioned such that the object intercepts it at the same point and the transverse position is determined with a mechanical detector. Therefore, the device of the '229 patent suffers the same drawbacks as the '043 patent, namely, (i) the focal length of the technoscope must be known, (ii) two measurements are needed, during which the technoscope must be fixed with respect to the object, (iii) limitations in the gradations of the measurement scale introduces errors, and (iv) the measurement scale must be manually read.
Known devices also use a magnification scale ring arranged adjacent to a focusing control ring having an indicator RV, for determining the magnification of the scone. Based on the position of the indicator of the focusing control ring with respect to the magnification scale, the magnification of the scope at that object distance is determined. Unfortunately, similar to the probe penetration knob in the system discussed in the '594 patent, such devices require magnification values to be manually read from a scale on the magnification barrel. Since such scales do not have fine gradations and since the magnification values must be manually read, errors are introduced. Even if the scale had fine markings, its diameter would have to be huge to have thousands of distinct "markings."
When such a scope is equipped with a graticule and a diopter focus control, this known device can also be used to determine the size of a viewed object. A graticule is a scale etched into a surface of a transparent glass plate included in the optical system. The diopter focus control is used to focus the graticule scale. An object is then focused by means of the focus control. Based on the number of graticules covered by the object image and based on the magnification level, the object size is determined. Unfortunately, the graticule scale is manually read which introduces errors. Manually reading the number of graticules covered by the object image also fatigues the user's eye. Moreover, since the number of markings on the graticule is limited, the accuracy is also limited. Furthermore, this method is inaccurate because the eye will accommodate an "out of focus", focus barrel position.
U.S. Pat. No. 4,558,691 (hereinafter "the '691 patent") discloses an endoscope in which an actual size of an observed object, a magnification of the scope, and an object distance can be determined based on a positional relationship between an indicating index and a stationary reference index. The indicating index is formed on a glass plate which moves up and down, perpendicular to the optical axis, as the lens barrel of the optical system moves back and forth along the optical axis. Unfortunately, as with the devices discussed above, the indicating index included in the device described in the '691 patent does not have fine gradations and must be manually read. This not only permits errors to be introduced, but also fatigues the eye of a user.
Japanese Patent Publication No. 5-288988 (hereinafter "the '988 publication") discusses the use of an encoder for determining changes in the magnification of a zoom lens system. However, this system is to be used for viewing objects at a fixed distance. That is, the encoder in the '988 publication determines changes in magnification of the scope but cannot determine the initial magnification of the scope and cannot determine object distance.
In view of the above described problems with existing scope measurement systems, a system for automatically measuring objects, with high resolution, is needed.